1. Field of the Invention
The present invention relates to a heterojunction bipolar transistor and a fabricating method thereof, and in particular, to those used for microwave devices (such as mobile telephone units) and optical communication signal processing circuits.
2. Prior Arts
FIG. 1 is a sectional view showing the structure of a heterojunction bipolar transistor (hereinafter referred. to as HBT) disclosed in xe2x80x9cExtended Abstracts of the 1992 International Conference on Solid State Devices and Materials, pages 316-318 and xe2x80x9cElectronic Lettersxe2x80x9d, Vol. 28, No. 25, pages 2308-2309. As shown in FIG. 1, in the HBT according to the first prior art reference, a sub-collector layer 52 is grown on a substrate 51. A collector layer 53 is grown on the sub-collector layer 52. A base layer 54 is grown on the collector layer 53. A part of the base layer 54 is coated with a very thin emitter layer 55. The thickness of the emitter layer 55 is around 30 nm. An emitter contact layer 56 is grown on the emitter layer 55.
A base electrode 61 is formed on an alloyed region 63. The alloyed region 63 is formed by heat treating the emitter layer 55. The base electrode 61 and the base layer 54 are ohmic contacted through the alloyed region 63. A part of the emitter layer 55 without being covered by the emitter contact layer is depleted and acts as guard-ring. Tile guard-ring 70 is depleted by pinning the Fermi level on front surface of a semiconductor so as prevent carriers from being recombined in the vicinity of the front surface of the base layer 54.
An emitter electrode 62 is formed on the emitter contact layer 56. An insulator film 80 is formed so that the insulator film 80 coats the guard-ring 70, the base electrode 61, the base layer 54, and the sub-collector layer 52.
However, in the HBT shown in FIG. 1, since the thickness of the emitter layer 55 is around 30 nm, even if the alloyed region 63 is formed, the contact resistance between the base layer 54 and the base electrode 61 becomes large. Thus, the high frequency characteristic of the HBT deteriorates.
As a second prior art reference that solves such a disadvantage, an HBT that does not have an alloyed region 63 is disclosed in JPA 10-303214.
FIG. 2 is a sectional view showing the structure of the HBT described in the publication as the second prior art reference. For simplicity, in FIG. 2, similar portions to those in FIG. 1 are denoted by similar reference numerals. In the HBT shown in FIG. 2, a base electrode 61 is formed so that the base electrode 61 contacts to a base layer 54 and a part of a guard-ring 70. Thus, the base electrode 61 and the base layer 54 are contacted without need to use an alloyed region 63. As a result, since the contact resistance between the base layer 54 and the base electrode 61 becomes small, the high frequency characteristic of the HBT can be suppressed from deteriorating.
However, in the HBT shown in FIG. 2, since the base electrode 61 is contacted to the guard-ring 70, when the HBT is driven, the surface potential of the guard-ring 70 varies. Thus, a part of the guard-ring 70 is not depleted, but becomes a neutral region. Consequently, a PN junction between the guard-ring 70 and the base layer 54 causes carriers to be recombined. As a result, since a leak current flows between the base and the emitter of the HBT, the current gain decreases.
On the other hand, when the base electrode 61 is not contacted to the guard-ring 70, carriers are recombined in the vicinity of the front surface of the base layer 54 and an edge portion 70a of the guard-ring 70. As a result, a leak current flows.
In order to overcome the aforementioned disadvantages, the present invention has been made and accordingly, has an object to provide an HBT and its fabricating method in which a contact resistance between the base layer and the base electrode is small and a leak current between the base and the emitter is suppressed.
According to an aspect of the present invention, there is provided a bipolar transistor, comprising: a collector layer of first conduction type; a base layer of second conduction type, formed on the collector layer; a prevention layer, formed on the base layer, for preventing carrier recombination; an emitter layer of first conduction type, formed on a first part of the prevention layer; and a base electrode, formed on a second part separated from the first part of the prevention layer.
The bipolar transistor may further comprise: a guard ring, formed on an outer periphery of the emitter layer and on the prevention layer.
The bipolar transistor may further comprise: an emitter electrode, formed on the emitter layer.
The bipolar transistor may further comprise: an emitter contact layer, formed between the emitter layer and the emitter electrode.
The bipolar transistor may further comprise: a sub-collector layer of first conduction type, formed below the collector layer.
The bipolar transistor may further comprise: a collector electrode, connected to the sub-collector layer.
The bipolar transistor may further comprise: an insulation side wall, formed on the guard-ring, for defining the size of the guard-ring.
The bipolar transistor may further comprise: an alloyed region, piercing the prevention layer, formed between the base layer and the base electrode.
The bipolar transistor may further comprise: a diffused region, piercing the prevention layer, formed between the base layer and the base electrode.
In the bipolar transistor, the emitter layer may be a III-V group compound semiconductor.
In the bipolar transistor, a V group element of the III-V group compound semiconductor may be phosphorus.
In the bipolar transistor, the III-V group compound semiconductor may be selected from a group consisting of InGaP, InGaAsP, InGaAlP, InGaAlAsP, AlGaP, AlGaAsP, GaP, GasP, and InP.
In the bipolar transistor, the emitter layer may have a laminate structure.
In the bipolar transistor, the laminate structure may have a sloped-composition layer between two adjacent layers.
In the bipolar transistor, the base layer may be a III-V group compound semiconductor.
In the bipolar transistor, a V group element of the III-V group compound semiconductor may be arsenic.
In the bipolar transistor, the III-V group compound semiconductor may be selected from a group consisting of GaAs, AlGaAs, InGaAs, InGaAs, InAlGaAs, and InAlAs.
In the bipolar transistor, the base layer may have a laminate structure.
In the bipolar transistor, the laminate structure may have a sloped-composition layer between two adjacent layers.
In the bipolar transistor, the prevention layer may be of first conduction type.
In the bipolar transistor, a potential gap of the prevention layer may be larger than a potential gap of the base layer so that the potential gap of the prevention layer functions as a potential barrier for majority carriers of the base layer.
In the bipolar transistor, when the bipolar transistor is driven, a portion of the prevention layer which is not coated with the emitter layer may be fully depleted.
In the bipolar transistor, when the bipolar transistor is driven, a portion of the prevention layer which is not coated with the emitter layer or the guard-ring layer may be fully depleted.
In the bipolar transistor, a potential barrier of the prevention layer for majority carriers of the emitter layer may be 100 meV or less.
In the bipolar transistor, the prevention layer may have a laminate structure.
In the bipolar transistor, the laminate structure may have a sloped-composition layer between two adjacent layers.
In the bipolar transistor, the alloyed region may contain palladium or platinum.
In the bipolar transistor, the diffusion region may be doped with at least one of zinc, beryllium, carbon, magnesium, and manganese.
In the bipolar transistor, the highest portion of the prevention layer may have an etching resistance against an etchant of the emitter layer.
In the bipolar transistor, the highest portion of the base layer may have an etching resistance against an etchant of the prevention layer.
These and other objects, features and advantages of the present invention will become more apparent in light of the following detailed description of the best modes of embodiment thereof, as illustrated in the accompanying drawings.